The detonation of a nuclear weapon produces radiating outputs which completely destroy all things in a zone near the center of the detonation and radiate outwardly therefrom with lesser destructive effect. The extent of the destruction zone and the destruction outside the zone are highly dependent upon weapon yield. Among these outputs which radiate from the point of detonation are x-rays, prompt and delayed gamma rays, neutrons, heat and mechanical shock. The first output to reach a remote location, which may be, for example, several miles outside the destruction zone, is a pulse of ionizing radiation which has the potential of damaging or destroying electronic components. Critical command, communication, control and intelligence systems must be able to function after the initial radiation pulse dissipates. While the destruction of electronic devices due to ionizing radiation is not as visually apparent as destruction by intense heat and air blast, such destruction may be just as complete and must be circumvented.
Semiconductor devices are formed of p-type material, having excess holes, and n-type material, having excess electrons. When p-type material and n-type material are brought together, a junction is formed with a depletion region about the junction. In the case of a diode, forward biasing of the device narrows the depletion region, causing the diode to act as a closed switch. On the other hand, reverse biasing of the diode results in widening of the depletion region, causing the diode to function as an open switch. Ionizing radiation such as gamma radiation generates excess electron-hole pairs within all semiconductor materials, resulting in transient leakage currents, commonly called photocurrents, that appear across reverse-biased semiconductor junctions. The high, transient photocurrents interfere with normal circuit operations and often result in permanent damage to electronic equipment. Interference with normal circuit operations without permanent damage to the device is referred to as upset, whereas permanent damage occasioned by dissipation of energy in high, transient photocurrents over a period of time is referred to as burnout.
The concept of circumvention encompasses all elements of design necessary to prevent upset or burnout resulting from ionizing radiation pulses. When circumvention is incorporated as part of a system hardening approach, both hardware and software requirements can be minimized. For example, consider a system that contains critical data that cannot be lost without dire consequences. By using circumvention, only the specific circuits containing the critical data must be hardened against upset. An ionizing pulse can be detected and used to trigger circumvention hardware to provide protection. If circumvention is not employed, the hardware must all be designed using circuit topologies and components that will resist upset at the maximum ionizing dose rate. The circumvention hardware can control the interfaces between upset-hardened circuits and non-upset-hardened circuits to prevent transients from the latter from entering the upset-hardened circuits. The use of circumvention thus reduces or eliminates the upset-hardening requirements for the majority of the system components thereby minimizing the cost of nuclear hardening of the overall system.
The heart of any circumvention system is a nuclear event detector which functions to sense ionizing radiation and quickly triggers protection circuitry. For a complex system, the sequence of events starts with the detection of the ionizing pulse. The detection signal then triggers a timer which controls the onset and pulsewidth of a circumvention signal. The pulsewidth of the circumvention signal must be longer than the longest hardware recovery time in the system, because the termination of this signal is used to initiate recovery, at which time the hardware must be functional. Any delay through the sensing and pulse timing network must be minimized because the hardware which prevents upset will be driven by this signal. At the initial edge of this signal, the upset prevention circuitry will be triggered. There are four basic functions that are to be inhibited at this point. The writing into hardened memory is halted, data processing is stopped, the over/under voltage and over-current shutdown circuitry for any power supply that must operate through the critical period is disabled and other critical outputs are prevented from upsetting.
At the initial edge of the output signal, burnout prevention circuitry can also be triggered. Because burnout requires a longer time to occur, this circuitry need not react as rapidly as the upset prevention circuitry. Burnout prevention hardware normally consists of power supply crowbars, other power supply switches and signal clamps to prevent the generation of damaging photocurrents.
After the initial signal edge, all of the circumvention hardware has been triggered and is performing its function. The most time-consuming function during this period is the crowbarring of power supplies due to the finite time required to discharge storage capacitors. At the terminating edge of the circumvention signal, all of the circumvention hardware is disabled, and both hardware and software recovery are initiated. During this restart, hardware recovery consists primarily of restarting any power supplies that were shut down and/or crowbarred. Software recovery consists of evaluating data integrity, initiating error correction algorithms, initializing registers and/or discrete outputs and restarting processors.
It is common for military systems, such as air launched cruise missiles, to include nuclear event detectors which are designed to provide an output signal in the form of a square wave of predetermined duration in response to sensing a nuclear event. One prior art nuclear event detector includes a sensing circuit incorporating a PIN diode which functions as a solid state ionization chamber. The sensor circuit provides an output which is amplified and used to trigger a timer circuit. This prior art nuclear event detector, which is formed of discrete components mounted on a circuit board, has to be custom designed for each application. Electrical components for setting the threshold level of the sensor circuit and the duration of timer pulse are also mounted on the circuit board. The resulting relatively large custom circuit board design is not conducive to pretesting. Accordingly, the prior art nuclear event detector is required to be tested after it is incorporated into the military system it is custom designed to protect. While this prior art detector has provided satisfactory nuclear hardness in specific applications, an increase in hardness is desirable for universal application.